Do you feel that OBT loading is worthwhile?

[fatelvis]

Here's a calculator : https://weatherby.dk/OBT/obt.html

What does the “node value” mean, and where can I find it for my particular barrel?

 

[CF Junkie]

Interesting that it has no place to load the barrel steel.
Seems to have missed the basic premise of OBT - the steel reflection speed sets the OBT.

 

[jimmymac]

With my .223 12 FV Savage with a Shilen Select Match barrel and a MDT LSS chassis, I have been averaging under 0.220 for over 500 5-round groups and 30% of my groups are under 0.200.

With results like that, you should consider using a BR rifle and go play with this crowd.


https://forum.accurateshooter.com/threads/shamrock-results.4175457/

 

[kelbro]

What does the “node value” mean, and where can I find it for my particular barrel?

Enter the number one in there.

 

[kelbro]

Interesting that it has no place to load the barrel steel.
Seems to have missed the basic premise of OBT - the steel reflection speed sets the OBT.

I would guess that it's some sort of average. Every batch of steel will have a slightly different makeup. There are very few absolutes in smelting and extruding. Even less in the rolling processes.

Chris Long published the theory and he might be able to provide more data.

 

[CF Junkie]

Based upon my calculations, Node 1 seems to be the 8th reflection and each additional node is a succeeding even reflection, so Node 3 is the 10th and Node 5 is the 12th.

The OBT numbers are too low for .3% carbon steel (19,107 fps) and too high for Vanadium (chrome alloy) steel (19,969 fps).
It looks like the steel reflection speed being used in their calculations is around 19,556 fps. I don't know of any barrel steel that has that reflection speed.

According to my calculations, the reflection positions for a 26-inch barrel would be off at the 12th reflection by the following:
QuickLOAD projection for the 12th reflection with a 26-inch barrel (Node 5 using 1 as the entry number)
was 1.3295 msec.
My calculations based upon the materials handbook values are:
.3% carbon steel (19,107 fps)= 1.3608 msec - 27.6% off the chamber moving toward the muzzle
Vanadium (chrome steel - 4140 and 4150) (19,969 fps) = 1.3020 - 25.3% off the chamber moving away from the muzzle
Stainless steel (20,000 fps)= 1.3000 - 27.2% off the chamber moving away from the muzzle
416R Stainless steel (20,014 fps) = 1.2991 - 28.1% off the chamber moving away from the muzzle

 

[kelbro]

I remember many years ago seeing a 'smith in Houston using a set of 'hangers' (I think they were for ceiling tiles) to determine the optimum barrel length. Barrel chamber end was clamped in a vise. He hung the inverted 'U' hangers over the barrel and then rapped the barrel with a wrench to 'ring' it. The hangers moved down the barrel and it supposedly showed resonance where the nodes were. Where the hanger stopped at the muzzle end was where he cut the barrel.

That was MANY years ago and I may not have remembered the process exactly but the group of guys that hung around his shop were benchrest guys and wildcatters so I thought he must know what he was doing.

 

[gunsandgunsmithing]

I remember many years ago seeing a 'smith in Houston using a set of 'hangers' (I think they were for ceiling tiles) to determine the optimum barrel length. Barrel chamber end was clamped in a vise. He hung the inverted 'U' hangers over the barrel and then rapped the barrel with a wrench to 'ring' it. The hangers moved down the barrel and it supposedly showed resonance where the nodes were. Where the hanger stopped at the muzzle end was where he cut the barrel.

That was MANY years ago and I may not have remembered the process exactly but the group of guys that hung around his shop were benchrest guys and wildcatters so I thought he must know what he was doing.

That sounds like he was finding vibrational "nodes" but the thing is, ya can't move an actual node to the end of a cantilevered beam(barrel). That's why we tune to where bullet exit happens at an anti-node...fwiw. It's physics...not something I'm pulling out of the air.

 

[Dgd6mm]

This makes for some really good reading.

 

[CF Junkie]

I have had 8 Savage barrels with .3% carbon steel and 2 Shilen 416R stainless barrels. I also have one 416R barrels that came with a Les Baer Super Varmint .223
The first Savage barrel was shot out by 2010 and I measured the length to the tip, not to the crown so it measured a bit longer than if I had measured it correctly. I don't know how far off it was because that barrel was replaced and is no longer around to remeasure.
The other 10 barrels were measured more accurately to the crown.

Of the 11 barrels, all but one had their best accuracy within +/- 0.001 msec of my calculated exit times using 19,107 fps reflection speed for the 8 Savage .3% carbon steel barrels and 20,014 fps for the 416Rs.
That would indicate that variations in lots of steel are negligible in changing results.
The original barrel's accuracy point was off by 0.003 msec. That was the one measured to the tip instead of the crown. (A 1/16 inch difference in measurement would produce a 0.0035 difference in exit time so that change seems within the range of error measured, especially if the relative changes are non-linear.)

Those 11 rifle barrels have amassed over 14,000 measured groups that I have carefully measured and logged and then used for calculating what produces the best accuracy.
Not every group has been great, but generally, the SD among my group sizes is about 0.030 or better.

My results have convinced me that my calculations for OBT are about as accurate as I can get them.
The difference of the reflection speed between the 8 Savage barrels .3% carbon steel and the 416R stainless steel barrels is 907 fps, that amounts to a difference between 0.10467 and 0.09993 msec., or 0.00474 msec., per pass down a 24-inch barrel, 0.00949 to get back to the chamber, or 0.05690 to complete the 12th reflection.

I have measured group sizes with the reflection at the muzzle and at the chamber. The group average is about 0.090 inches smaller with the reflection at the chamber versus the muzzle. (Given my induced accuracy variations, I would estimate that the difference could vary.)
That amounts to about 0.0375 inch per inch of barrel reflection. For a 24-inch .3% carbon steel barrel the difference in reflection is 0.10467 msec. or 0.043625 msec. per inch.
I am not sure the results are perfectly linear along the barrel but a reflection that is off the chamber so a 25% difference in reflection amounts to 0.0234 inches in group size accuracy. When you shoot groups that average under 1/4 inch at 100 yards, that kind of loss in accuracy is visible.

Most of my measurements have been focused at the 12th reflection.
I have measured accuracy at the 10th, 12th and 14th reflections and depending on the caliber, the results are relatively the same.
Most heavier calibers are over Pmax at the 10th reflection so the 10th reflection tends to only have measurements for light bullets with light calibers.
Most calibers support the 12th reflection so I tend to concentrate comparisons at that reflection.
Some calibers are under minimum pressure recommendations at the 14th reflection, so that also limits the collection efforts..

I have noticed a slight accuracy preference with the 6.5mm CMs and .308s for the 12th reflection over the 14th reflection, but not so much for .223s.
Perhaps that effect might be related to maintaining bullet stability at slower velocities, but with heavy bullets, especially with the heavier calibers, the faster reflections tend to wear out the primer pockets on the brass more quickly.
The 10th reflection is pretty much out of the picture for the 6.5mm CM and .308 calibers because the loads are over Pmax with my barrel lengths even with lighter bullets.

 

[CharlieNC]

That sounds like he was finding vibrational "nodes" but the thing is, ya can't move an actual node to the end of a cantilevered beam(barrel). That's why we tune to where bullet exit happens at an anti-node...fwiw. It's physics...not something I'm pulling out of the air.

Yep. And OBT presumed the accuracy node was because the diameter at the muzzle was more stable at that point due to the traverse wave propagation; but ladder tests demonstrate that while the frequency is correct the mechanism is because the muzzle vibrates vertically due to the disturbance which causes the vertical poi wave to enable positive compensation.